The invention is directed towards an improved security document and, more particularly, to a security document having a non-orthogonal array as the underlying location pattern for image elements.
Color photocopiers have been used for years to make accurate copies of commonly available documents. In many cases, there are legitimate reasons for making such copies. Unfortunately, color copies may also be made and used for illegal purposes. Specifically, there has been concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, birth certificates, college transcripts, prescriptions, and other documents of value, for illegal purposes.
Many techniques have been developed to prevent improper reproduction of security documents. One of the most successful is the use of a hidden warning message which is readily apparent on reproduced copies of a document, but which is invisible, or nearly so, on the original document. While this does not actually prevent a copy from being made, it does make it apparent that the copy is not the original document.
Many techniques have been used to produce this effect. One technique is shown in U.S. Pat. No. 4,227,720. This patent discloses using small dots in a conventional orthogonal halftone dot array as background elements, while larger dots in a similar orthogonal halftone dot array are used to form a warning word. A single tone warning phrase and a single tone background pattern are used. Tone refers to the visual effect produced by solid ink coverage or by halftone dots, bars, marks, or other elements which cover a portion of a printed area and which usually have a frequency that is measured in dots, lines, marks, or other elements per inch. Halftone dots, bars, marks, or other elements printed with a dark ink may be more or less uniformly distributed over an area to produce the visual effect of a lighter overall color than the color of the ink. Printing an image with less than full area coverage is said to be printing less than a full tone image. The warning phrase and background pattern area tones are of different frequency and are made up of dots, bars, or marks of differing size, but they are selected to provide similar appearance to the eye of a casual observer. A less than full tone effect may also be produced by full area coverage of a paler color of ink than the darker color of ink used for the halftone dots, bars, or marks.
Because the tone of the warning phrase and the tone of the background pattern are selected to be generally the same, these two areas have much the same visual impact on an observer of the original document, and the warning phrase is not readily perceived. The optics of color copiers have typically been unable to reproduce relatively small halftone dots, lines or other elements. As a consequence, reproduced copies of the original document will have a noticeable warning phrase.
As taught in the '720 patent, a camouflage pattern is often utilized to obscure the warning phrase further. The camouflage pattern may be defined by areas in which the dots, bars, or marks have been deleted from both the warning phrase and the background pattern. The camouflage pattern may also be defined by a pattern of dots, bars, or marks which are smaller than or larger than those used in the background pattern and the warning phrase, or by areas of complete coverage of a paler ink. The camouflage pattern confuses the eye of the casual observer so that the warning phrase is not readily apparent, even if the tone of the warning phrase and the tone of the background pattern differ somewhat.
In recent years, color copiers have been improved substantially, and these improvements have made the above techniques less effective in protecting documents. By manipulating the control settings on improved copiers, copies can be made of documents carrying a hidden warning phrase in which the warning phrase does not appear on reproductions when some of the most commonly used frequency and size combinations are used. For example, by adjusting the settings for sharpness and lightness/darkness it has still been possible on some copiers for a skilled operator to produce a copy in which the warning phrase is not visible. Furthermore, different orientations of the document on a copier can affect the visibility of a warning phrase on the resulting copy. This is especially true when halftone lines are used instead of dots.
Therefore, there remains a need in the art for a security document which provides improved protection against copying over a wide range of copier settings and document orientations.